The process of coring subterrain formations typically involves drilling down to the point of interest with a conventional drilling assembly including a drill bit, this is well known in the art. The depth where coring is to commence is typically determined by analyzing drill cuttings collected at surface from the drilling process and/or results from logging sensors that are used to measure formation properties during the drilling process, known as Measurement While Drilling (MWD) systems. The drill cuttings are transported to the surface by means of the return mud flow, this may typically take 30 minutes or more. The sensors of the MWD system, typically capable of measuring natural radiation from the formation, i.e. Gamma Ray this is a parameter of natural gamma radiation of the formation, and electrical conductivity, i.e. Resistivity which is a parameter of inverted electrical conductivity of the formation, is placed some distance behind the drill bit. This means that both sources of information represent formation that has already been drilled, so the uppermost part of the formation that is wanted to be cored is quite often missed.
Once the point of interest is determined, it is typically pulled out of the drilling hole to replace the drilling assembly with a coring assembly. The coring assembly, consisting of a hollow core bit and an inner string for collecting the core is run into the drilling hole and coring of the formation of interest is carried out. Upon completion of the coring process, the core assembly is pulled out of the drilling hole to retrieve the inner string containing the core. Subsequently, a new coring assembly is run in the drilling hole to continue coring, or a drilling assembly is run in the drilling hole to revert to drilling mode, where no core is collected. The complete process includes minimum two roundtrips from the bottom of the drilling hole to surface to first pick up and run a coring assembly for coring, then to change back to a drilling assembly for drilling. This takes substantial time and also increase risk of the wellbore conditions to deteriorate, giving potential problems as drilling continue.
It would be desired from a time, cost and wellbore quality point of view to be able to both cut and preserve the core without having to trip the bottom hole assembly out of the wellbore after coring is completed. One relevant coring system has been described in U.S. Pat. No. 5,568,838 on a Bit-stabilized combination coring and drilling system. In this system a specially designed combination drilling and coring bit including a retrievable center plug is used to alternate between drilling and coring modes. After coring, the core is retrieved by lowering a catch mechanism on a wireline inside the drillpipe, engaging the top of the core barrel and retrieving the core assembly by means of the wireline. This has the advantage of not requiring a roundtrip to surface with the coring assembly. However, it still requires lowering the wireline down to the core barrel and pulling out to retrieve the core at surface. This takes time and also has limitations if the borehole inclination (i.e. the angle of borehole relative to vertical) is high, thus limiting the ability of the wireline assembly to travel to the bottom of the wellbore by its own weight. Also this method represent a risk that the core assembly may get stuck and the wireline broken during the retrieval process, or not being able to engage the core with the wireline catch mechanism, both resulting time consuming operations to retrieve the core and revert to drilling mode.
Furthermore, during normal coring operations the core is cut and subsequent retrieved by tripping the coring assembly all the way out of the drilling hole to surface. During the trip to surface the core will be subject to lower pressures and temperatures. This causes gases and liquids present within the core to bleed out of the core sample. Vital information about the chemical material within the core is lost as it escapes from the core during transport to surface, and the core sample will not be representative of the downhole formations from where it was cut.
Pressure core systems have been developed where the core is collected in a core barrel which is sealed off after the core is cut to provide a pressure-tight seal prior to retrieving the core to surface. It may involve a self-contained high pressure nitrogen gas supply with a controlled expansion of an accumulator compartment to maintain approximate formation pressure (a parameter of the virgin pressure of the formation), trapped in the pressure-tight compartment of the barrel, ref. U.S. Pat. No. 3,548,958 issued to Blackwell et al. Pressure core systems typically also include flushing of the core, either on surface or downhole, with the disadvantage of potentially contaminating the core with the flushing fluid. Furthermore, handling of the core at surface both include risk due to the pressure contained within the mechanical compartment and the requirement of freezing the core and maintaining it in a frozen state during transport to the laboratory.
One such pressure core system also include a non-invading gel as is described in U.S. Pat. No. 5,482,123 issued to Baker Hughes Incorporated. The non-invading gel will reduce the invasion of mud filtrate into the core during the coring process. As the non-invading gel is not pressure tight it will not be capable of fully preventing material from within the core of escaping as pressure is lowered during travel from downhole to the surface, and only partly be capable of preserving the core in a relatively pristine state. Also, as the core barrel needs to be filled with the non-invading gel prior to running it in the drilling hole, the amount of non-invading gel relative to the volume of the core after it has been cut may be substantial. For instance, if it is planned to cut a 10 meter core, but only 1 meter core is cut prior to it for operational reasons need to be retrieved, the volume of non-invading gel that may interact with the core is substantial. Also, the non-invading gel surrounds the core material during the whole process of cutting the core, while the current invention encapsulate the core during or after the coring process is completed, minimizing the time allowed for interaction between the core and the non-invading gel.
The present invention relates to a method and apparatus for overcoming shortcomings of prior art when cutting and retrieving a core to be analyzed.
The method and apparatus for cutting a core and encapsulating it for later analysis is described by receiving the core in a core barrel, encapsulating the core at downhole conditions with a material capable of providing a pressure tight seal around the core, temporary storing the core downhole within the core barrel and subsequently retrieving the core at the surface for analysis, later referred to as the coring mode. Furthermore the invention includes sensor technology for measuring the characteristics of the core downhole during the coring process, transmitting said information to surface for analysis and using said information to identify sections of the core that is required to be collected, encapsulated, stored and subsequently retrieved for analysis. The system may include downhole intelligence to allow said identification of wanted core intervals to be determined downhole. Last the invention includes apparatus for grinding away unwanted core material of formations of no interest and removing the same by discharging this material in the return mudflow, later referred to as the drilling mode.
The present invention can be used for all or any operations where a subsurface core sample is required.